Control system



Jan. 1, 1935. M. A. EDWARDS 1,985,981

' CONTROL SYSTEM 7 Filed Dec. 10, 1932 5 SheetS-Sheet 1 20 Fig.1.

Invent-or: Martin A. Edwards,

by MW His Attorney.

Jan. 1, 1935. MIA. EDWARDS 1,985,931

CONTROL SYSTEM 7 Filed Deq. 10, 1932 IS' SheetS-Sheet 2 Fig.2;

Inventorf: Martin A. Edwards,

' His Attorne g.

Jan. 1, 1935. M. A. EDWARDS 1,935,981

' CONTROL SYSTEM Filed D90. 10, 1932 Sheets- Sheet 3 Fig.5.

Ihventoh: Martin A. Fdwards,

Hi 5 Atbdrne g.

PatentcdJan. 1 1935 PATENT OFFICE CONTROL SYSTEM Martin A. Edwards,Schenectady, N. Y., assignor to General Electric Company, a. corporationof New York Application December 10, 1982, Serial No. 646,670

3- Claims.

This invention relates to control systems, more particularly tofollow-up control systems for driving an object into positionalagreement with a control device, and it has for an object the provisionof a simple reliable and improved system of this character.

More specifically the invention relates to follow-up systems in whichelectric valve apparatus is employed to control the supply of current tothe driving motor and in which fine and coarse controlling means areemployed for controlling the valve apparatus, and a specific object ofthe invention is the provision of improved meansfor shifting the controlfrom the fine controlling means to the coarse controlling means when thepositional disagreement of the control device and driven object exceed apredetermined amount.

In carrying the invention into effect in one form thereof, high speedrotary induction apparatus supplies the controlling voltage to the gridof the electric valve apparatus to provide fine control of the systemwhen the positional disagreement of the pilot device and driven objectis small, and additional rotary induction apparatus operating at lowspeed provides the coarse control, and means are provided for shiftingthe grid control from the high speed to the low speed rotary inductionapparatus when the positional disagreement of the pilot device and'thedriven object exceeds a predetermined amount.

In illustrating the invention in one form thereof, it is shown asapplied in a gun directing system,

- but it will of course be understood that the inven tion has otherapplications,

For a better and more complete understanding of my invention referenceshould now be had to the following specification and to the accompanyingdrawings in which Fig. 1 is a simple diagrammatical representation of anembodiment of the invention and Figs. 2 and 3 are simple diagrammaticalrepresentations of modifications. Only the modification of Fig. 2 isclaimed in the present application. The modifications of Figs. 1 and ,3arecIaimed ih copending application, Serial No. 716,618, filed March 21,1934, which is a continuation in part of the present application and isassigned to the. assignee of the present invention.

Referring now to the drawings, an object such forexample asthatrepresented by the gun 10 is driven into positional agreement with apilot or control device represented as a telescope 11 by suitabledriving means such for example as rep resented by the direct currentelectric motor 12, to the drive shaft of which the gun mounting plat-(Cl. 172 -239) I form is connected by means of'suitable reductiongearing 13. Direct current is supplied to the armature oi the motor 12by means of electric valve apparatus 14 illustrated as comprising a pairof valves 15 for supplying current in one direction to the armature ofthe motor and a second pair of valves 16 for supplying current to themotor armature in the opposite direction, to which the valve apparatusis connected by means of conductors l7.

The electric valve apparatus 14 is in turn supplied through atransformer 18 from a suitable source of alternating voltage representedin the drawings by the three supply lines 20 through the upper andmiddle lines of whiclrthe terminals of the primary winding of thetransformer are connected as illustrated and to the terminals of therespective secondary windings of which the anodes of the pairs of valves15 and 16 are connected as illustrated. Although the electric valves maybe of any suitable type, they are preferably of the three electrode typeinto the envelope of which a small quantity of an inert gas *full waverectification, that is to say the anodes of each pair of valves areconnected to opposite terminals of the associated secondary winding ofthe supply transformer whilst the cathodes of each pair of valves isconnected to one of the supply conductors 1'1 and the'mid point of theassociated supply transformer secondary winding is connected to theother conductor 17; Alternating voltage is supplied to the grids of thevalve apparatus by means of grid supply transformers 21 and 22, theprimary windings of which are connected in series relationship with eachother and with the secondary winding of the grid bias transformer 23,whilst the opposite terminals of the secondary windings of eachof thegrid transformers are respectively connected to the grids of theassociated pair of electric valves. One terminal of the primary windingof the grid bias transformer 23 is connected to the lower supply line 20and the opposite terminal is connected to intermediate points of theprimary windings oi thesupply transformer 18 thereby giving the gridvoltage 0! the electric valve apparatus an initial phase bias withrespect to the anode voltage. This initial phase bias may be l anydesired amount but is preferablysubstantially 90 lag in The averagevalue of the current flowing in the output circuit of electric valveapparatus of the above described type is varied as desired by varyingthe phase relationship between the grid and anode voltages. For example,when the grid and anode voltages are substantially in phase with eachother, a current flowing in the output circuit is maximum and when thegrid andanode voltages are substantially 180 out of phase with eachother the current is minimum or zero whilst for intermediate phaserelationships thecurrent has corresponding intermediate value. In orderto vary the phase relationship between the grid and anode voltages acomponent voltage of variable magnitude is supplied to the grid circuitsubstantially in phase with the anode voltage by means of a transformer24, the opposite terminals oi the secondary winding of which arerespectively connected to a point between the primary winding of thegrid transformers hand and a mid-point of the secondary winding of thegrid bias transformer 23 and the opposite terminals of the primarywinding of which transformer are connected to the upper and middlesupply lines 20, by means of conductors 25, 26, and 27 to-- gether withrotary induction apparatus illustrated as comprising a rotary inductiondevice 28 referred to as the transmitter and a similar rotary inductiondevice 30 referred to as the receiver connected in circuit asillustrated. Therotary induction device 28 comprises a. rotor member 28aprovided with a single phase winding (not shown) and a statorziisprovided with a distributed three-element winding (not shown) that isphysically similar to the polyphase winding of the usual polynhasedynamo electric machine. The stator and rotor windings are arranged ininductive relationship with each other so that the alternating magneticfield due to the current flowing in the primary winding induces voltagesin the elements of the secondary winding. The receiver 30 is in allrespectsidentieal .with the transmitter 28 and the terminals of itsstator winding are connected to the terminals of the stator winding ofthe transmitter by means of conductors 31 so that the voltages inducedin the stator winding of the transmitter cause currents to flow in thestator winding of the receiver thereby producing a magnetic fieldsimilar to the magnetic field produced by the current flowing in therotor winding of the transmitter. Rotation of the rotor winding of thetransmitter causes a voltage to be induced in the rotor winding of thereceiver due to the rotation of the magnetic field of the receiver andthe magnitude of this induced voltage depends upon the relationship ofthe axis of this winding to the axis of the magnetic field, e. g. whenthe axes of the magnetic field and of the rotor winding are parallel theinduced voltage is maximum whilst when these axes are at right angleswith each other the induced voltage is zero. It will therefore be clearthat rotation of the rotor of the transmitter or of the receiver willvary the magnitude of the component voltage supplied to the grid circuitof the electric valve apparatus which in turn will result in a variationin the phase relationship of the resultant of the the telescope and therotor of the transmitter can be .made as large as is desired, forexample the ratio may be as much as 72:1, i. e., for each degree that.the telescope is rotated the rotor of the transmitter is rotated 72".The rotor of the receiver 30 is connected to the gun platform by meansof gearing 34 arid 35 having the same ratio as the gearing 32 and 33.

This large gear ratio provides a very fine and 'a very'accurate control.If the ratio is 72:1 as

assumed, then for'each of rotation of the telescope the rotor of thetransmitter 28 is rotated a full 360. However, since the axes of therotor winding of the receiver 30 and the magnetic field of the statorare parallel for each 180 of revolution of the rotor of the transmitter,it will be clear that the telescope and the gun must not be allowed tobecome more than 2%" out of correspondence witheach other because whenthis amount of positional disagreement occurs the same relationshipexists between the rotors of the-transmitter and receiver as exists whenthe telescope and gun are in correspondence. with each other. Inpractice, under actual operating conditions, the telescope often doesbecome more than this predetermined amount out of corre-v spondence withthe gun and a coarser system is therefore provided for taking over thecontrol of a high speed system before this maximum permissible amount ofpositional disagreement exists. This coarse system is illustrated ascomprising a transmitter 36' that is identicalwith the transmitter 28and a receiver 37 that ,is similar to the receiver 30 with the exceptionthat the stator of the receiver 37 is rotatably mounted in ballbearings. The single phase rotor windin of both the transmitter 36 andreceiver 3'7 are connected to the upper and middle supply lines 20 andtheir stator windings are connected to each other by means of theconductors 38. The rotor of the transmitter is directly connected to theturntable, upon which the telescope is mounted. by means of the bevelgearing 40 having a 1:1 ratio whilst the rotor member of the receiver3'! instead of being connected to the gun platform is free to rotatewhereas the ball bearing mounted stator of the receiver is geared to thegun platform by means of gearing 41 having the same- The shaft of therotor member of the receiver 37 is provided with a heart-shaped cam v42which when the system is in correspondence occupies the central orneutral'position in which it is illustrated. The movable contact arm ofa suitable switching device 43 is biased to a central position betweencooperating stationary contacts 44 and -15 by means of a spring and thecontact arm 43 is further provided with a cam roller 48 engaging the cam42 by means of which the contact arm 43 is moved into engagement withone or the other a of the stationary contacts 44, in response to mapperrotation ofthe cam 42 in one 'direction or the other as the Theswitchingapparatus illustrated as'comprising a pair' of electromagneticswitchingdevices 4'7 and 48 respectively are under the control ofthe'switching device 43 and are selectively energized and operated inresponse to actuation of the movable contact arm 43 vinto engagementwith'one or the other of the stationary contacts 44, 45.

For the purpose of supplying an alternatin voltage to the grid circuitof the electric valve apparatus when'the telescope and gun are more thanthe permissible "amount out of correspondence and, the fine controlsystem is inactive a transformer 50 is provided having its primarywinding connected to'the middle and upper supply lines and the mid-pointof its secondary winding connected to theleft-hand terminal of r theprimary winding of the grid supply transoperated to its upper closedposition. I

former 24 and one orthe other of the opposite terminals .of thesecondary winding of the transformer 50. is connected to the right handterminal,

'of the pr m y winding of the grid supply transformer 24 depending uponwhich of the electromagnetic switching devices 47, 48 is energized andWith the above, understanding or the appstem the operation of the systemitself willr'eadily be understood from the following detaileddescription: f I

Assuming the telescope 11 and gunlo to be in positional agreement thesystem is in its normal deenergized condition in which it isillustrated.

The manual rotation of the telescope 11 eifects'a corresponding butmultiplied rotation of the rotor of the transmitter 28 causing arotation or the magnetic field of the stator of receiver 30 sothat avoltage is induced in the rotor winding of the receiver proportional tothe amount'of rotation of the telescope 11. This induced voltage isapplied to the grid circuit of the electric valve apparatus advancingthe phase relationship of the grid voltage ofv one pair of valveswithrespect to the anode voltage and retarding the voltage of the other pairof valves. Assuming that the direction of rotation' of the telescope issuch as to advance the phase of the grid voltage of the pair of valves15 current is supplied by this pair of valves to the armature oi thedriving motor 12 in such a direction that the gun 10 is caused to followthe movement of the telescope. The rotation of the telescope 11 alsoproduces a corresponding rotation of the rotor of the transmitter 36 anddue to a sellsynchronizing property of rotary induction apparatus ofthis character, the roto-- member of the receiver 3'! tends to' rotatein synchronism. with the rotor of the transmitter. The rotation oi thestator member of the receiver 37 by the.

motor 12 is opposite to the direction of rotation of the-rotor memberand consequently if the gun remains in synchronism with the telescope11, the rotor member of the receiver 3'? remains at rest in space withthe result that the cam member 42 also remains at rest in the positionillustrated.

However, if the gun 10 cannot tollow the rapid] movement of thetelescope and thetelescope and gun become more'than the maximumpermissible amount out of correspondence the heart cam' respondence issuch as to actuate the movable contact member 43 into engagement withthe stationary contact member 44 an energizing circuit is completed forthe operating coil of the electromagnetic switching device 4'7 that isreadily traced from the upper supply 'line20; through conductor 51,contacts 43 and 44, operating coil of switching g 30 and the gridcircuit of course renders the high i speed fine control system inactiveand the-con nection oi the transformer 50 tothe grid circuit causes avoltage to be app ied to the grid circuit such that the currentsuppliedhy the; valve ap paratus to the armature of themo'tor 12oausesthe motor 12 to drive the gun 10 at maximum speed in the same directionas that'fin' which'the 5 p p telescope 11 is moving. ratus and itsorganization in the completedsys Had the departure from correspondencebeen in the oppositedirection, the/cam member; 42

would have rotated the contact arm "43 intofem gagement with thestationary contact 45 and the electromagnetic switching device 48 wouldhave been actuated to its upperclosedposition to take the control awayfrom the high speed finecontrol system and to connect the upperterminals or the preaching correspondence with the telescope at maximumspeed and when it arrives within the predetermined maximum permissibleamount oi positional disagreement with the telescope the cam 42 is againin the central position in which it is illustrated and the contact arm43 is moved out of engagement with the stationary contacts 44 therebydeenergizing the electromagnetic switching device 47 which is respondingto the deenergization of its operating coil descends to'the lowerposition disconnecting the secondary oi the transformer 50 andreestablishing the connections from the receiver 30 tothe grid circuitthereby transferring the control back from the low speed coarse systemto the high speed fine system. Finally as the gun approaches exact andaccurate correspondence of the telescope 11 the axis of the-rotor of thereceiver 30 approaches a right angle relationship with the axis of themagnetic field of the stator and as a result the voltage induced in therotor winding becomes zero and the pair of electric valves 15 becomedeenergized and cease to supply current to the driving motor 12 so thatthe gun 10 is finally brought to restin corm respondence to thetelescope .11.

In the modification of Fig, 2 the telescope 58,

gun 5'1, high speed rotary induction apparatus 58, 60 and low speedrotary induction apparatus 61, 62 are all similar to thecorrespondingelements oi the system oLFig. 1 with the exception that thereceiver 62 of the low speed system has a fixed stator instead of arotatable stator as in Fig. 1. The gun driving motor 63 is illustratedas a series type motor provided with split series ileld windings 64 and65 supplied from a suitable source 66. A pair of valves 67 connected tothe terminals of the rotor winding of the receiver are under the controlof the high speed system so as to energize one or the other ofelectromagnetic switching devices 68, 70 so as to energize one or theother of the split field windings 64, and effect rotation of the motor63 in one direction or the other, depending upon which of the electricvalves 67 is energized. Although the valves 67 may be of any suitabletype they are preferably of the high vacuum pure electron dischargetype. Alternating voltage is supplied to the anodes of these valves froma suitable source 71 through a supply transformer 72 to the right handterminal of the secondary winding of which the anodes of both valves areconnected as illustrated with the operating windings of theelectromagnetic switching devices 68, respectively in circuit therewith.Current to heat the oath-- odes of these valves is supplied by themiddle portion of the secondary winding of the transformer 72 whilst asuitable negative direct current bias is applied to the grids of thevalves through the left hand portion of the secondary winding oftransformer, 72, copper oxide rectifying bridge 73 and potentiometer 74.As illustrated the grids of the valves 67 are respectively connected toopposite terminals of the secondary winding of the grid transformer 75the mid-point of which is connected by means of a movable contactengaging the potentiometer 74 to provide adjustment of the grid bias asdesired. Preferably the grid bias is adjusted to cut off, i. e. bothvalves are deenergized and both electromagnetic switching devices 68, 70are likewise deenergized and occupy the lower open positions in whichthey are illustrated. Similar electric valve apparatus 76 is under thecontrol of the low speed coarse system 61, 62 and electromagneticswitching devices 77, 78, the operating windings of which arerespectively connected in the plate or anode circuits of the valve 76,eifect connection of the armature of the motor 63 through one or theother of the split field windings 64, 65 to the source 66 and providerotation of the motor 63 in one direction or the other. Grid biasingmeans 80 similar in all respects to the grid biasing means described inconnection with theelectric valves 67 are provided for biasing the gridsof electric valves 76 to cut oi! so that both valves are normallydeenergized. The primary winding of the grid transformer 75 is connectedby means of conductors 82 to the rotor winding of the receiver 60 sothat valves 67 are under the control of the line high speed system andsimilarly the termi- .nals of the primary windings of the gridtransformer 81 are connected by means of the condoctors 83 to the rotorwinding of the receiver 62 so that the valves 76 are under the controlof the low speed coarse system.

In operation, the rotation of the telescope 56 eifects a greatlymultiplied rotation of the rotor of thetransmitter 58 and causes avoltage to be induced in the rotor winding of the receiver 60 andapplied through the transformer 75 to the grid of the electric valves67. The instantaneous polarity of this voltage of the terminals of thesecondary winding of transformer 75 depends of course upon the directionof rotation of the rotor member of the transmitter 58. Assuming that at.left hand terminal of the secondary winding of transformer 75 is alsopositive, the negative grid bias of the left hand valve 67 is overcomewhilst the negative grid bias of the right hand valve is still furtherincreased. This results in energization of the left hand valve causingcurrent to flow in the anode circuit through the operating coil of theelectromagnetic switching device 68, the movable contact member of whichis operated to the upper closed position in response to the energizationof the operating coil to complete a circuit for the armature of themotor 63 through the field winding 65 that is readily traced from theupper supply line 66, through conductors 84 and 85, contacts ofswitching device 68 in the upper closed position thereof, thence byconductors 86 to and through the split series field winding 65 andarmature of motor 63 to the lower supply line 66 thereby effectingrotation of the motor 63 in such a direction as to drive the gun 57 intopositional agreement with the telescope 56. If the telescope 56 becomesmore than the permissible maximum amount out of correspondence with thegun 57 the voltage induced in the rotor winding of the receiver 62becomes suflicient to overcome the negative grid bias of one of thevalves 76 (depending upon the direction of departure fromcorrespondence) thereby energizing one of the valves. Assuming that thedirection of departure from correspondence is the same as that whicheffects the energization of the left hand valve 67, the left hand valve76 will be energized and current caused to flow in the anode circuit ofthis valve through the operating winding of the electromagneticswitching device 77 and as a result of the energlzation of thisoperating coil the movable contact of the switch is moved to its upperposition in which it interrupts the connections 82 thereby deenergizingthe valve apparatus 67 and the entire high speed fine control system,andisimultaneously bridging its upper stationary contacts tomaintain themotor circuit previously traced. As a result, the gun 57 continues to bedriven at high speed under the control of the coarse low speed controlsystem. As the telescope is trained on the target and finally comes torest and the gun 57 arrives within the maximum permissible amount ofdisagreement with the telescope, the voltage induced in the rotorwinding of the receiver 62 is sufficiently decreased to render thenegative grid bias of the valve '76 effective again, therebydeenergizing the valve and the operating coil of the switching device77. The latter in response to the deenergization of its operating coilallows its movable contacts to descend from its upper to its lowerposition in which it reestablishes the connections from the receiver 60to the primary winding of the transformer 75,

thus retransferring the control to the low speed fine control system.

The motor finally is brought to rest with the gun in correspondence withthe telescope.

It will be clear that if the telescope had been rotated in the oppositedirection from that assumed in the above discussion, while the highspeed fine control system was active the right hand valve 67 would havebeen energized, the switching device 70 operated to its upper positionand the motor 63 caused to rotate in the opposite direction, and the gun'7 driven in the direction of the movement of the telescope. Similarly,while under the control of the low speed coarse system the right handvalve '76 would have been energized, the switching device 78 operated toits upper position, to transfer the control from the high speed to thelow speed system and the motor 63 caused to operate under the control ofthe switching device 78 until the gun 57 arrived within the maximumpermissible amount of positional disagreement.

In the modified system of Fig. 3, the switching device and the drivenobject are the same as illustrated in the previous figure. The highspeed rotary induction apparatus 91, 92 and the low speed rotaryinduction control apparatus 93, 94 are identical with the apparatus 58,60, 61, and 62 of Fig. 2 and for this reasonrequire no furtherdescription. Similarly the electric valve apparatus 95 of Fig. v3 isidentical with the valve apparatus 14 of Fig. l and the anode, grid, andcathode supply connections for the pairs of valves 96 and 97are-identical with those of the pairs of valves 15, 16 of Fig. 1. Theelectric valves 98,

99 are of the high vacuum type as are the valves 67 of Fig. 2 and areconnected in the same manner; the grids being biased to cut off bynegative grid bias means 100 similar in all respects to the grid biasingmeans 73 of Fig. 2.

With the telescope 101 and the gun 102 in correspondence both sets ofelectric valve apparatus 96, 97 and 98, 99 are deenergized and theelectromagnetic switching devices 103, 104 respec tively under thecontrol of valves 98, 99. are likewise deenergized and the movablecontacts cc.- cupy the lower positions in which they are illustrated. Amovement of the telescope effects a rotation of the rotor of the highspeed transmitter 91 resulting in a voltage being induced in the rotorwinding of the receiver 92 and applied to the grid circuit of theelectric valve apparatus 95 so that the phase of the grid voltage of oneof the pairs of valves is advanced and that of the other pair retarded.Assuming that the direction of rotation of the telescope is such as toadvance the grid phase of the pair of valves 96, current is supplied tothe gun driving motor 106 in a direction such that the gun 102 is drivenin a direction corresponding to the direction of movement of thetelescope.

If the telescope 101 becomes more than the maximum permissible amountout of correspondence with the gun, the voltage induced in the rotorwindings of the receiver 94 becomes sufficlently great to overcome thenegative grid bias of one of the valves 98, 99 which for the purpose ofillustration is assumed to be the valve 98. Simultaneously, the negativegrid bias of the valve 99 is increased. The removal of the negative biasfrom the grid of the valve 98 causes this valve to become conducting andto energize the operating winding of the switching device 103, themovable contact of which is attracted to its upper position in responseto the energization of its operating coil thereby interrupting theconnections from the high speed receiver 92 and the grid circuit of theelectric valve apparatus 95, thus rendering the high speed fine controlapparatus ineffective and at the same time the movable contact of theswitching'device 103 completes'the connections 108 from the left handprimary winding of the grid transformer 110 to the left hand of thesecondary winding of the supply transformer 111. The polarity of thevoltage thus applied to the grid circuit of electric valve apparatus 95is such that the pair of valves 96 is maintained energized and theeffective value of this voltage is substantially constant and of suchmagnitude that the gun driving motor 106 is caused to operate at maximumspeed under the control of the low speed coarse control system.

As the telescope 101 comes to rest trained on the target and the gun 102arrives within the maximum permissible positional disagreement with thetelescope, the voltage induced in the rotor winding of the receiver 94decreases suiiiciently to render the negative grid bias applied to thevalve 98 effective and to render the valves non-conducting. As a resultof this the switching device 103 is deenergized and its movable contactdescends to the lower position in which it is illustrated therebyinterrupting the connections 108 and taking the control away from thelow speed coarse system and simultaneously reestablishing theconnections 107 and retransferring the control the high speed finecontrolling system. Finally, as the gun 102 approaches ex act. andaccurate correspondence with the telescope the voltage induced in therotor of the receiver 92 becomes greatly reduced and the phase of thegrid voltage of the pair of valves 96 retarded thereby decreasing thecurrent supplied to the motor 106 to zero so that the gun is finallybrought to rest in correspondence with the telescope.

It will be clear that had the telescope 101 been rotated in the oppositedirection the pair of valves 97 would have been energized under thecontrol oi the high speed fine system and the motor 106 caused to drivethe gun 102 in the direction of movement of the telescope. Likewise ifthe positional disagreement oi the gun and telescope under thiscondition had exceeded the maximum permissible amount the valve 99 wouldhave been energized and the switching device 104 operated to its upperposition to interrupt the connections 107 and transfer the control fromthe high speed line system to the low speed coarse system with therighthalf of the secondary winding of the transformer 111 connected tothe primary winding of the grid transformer 110 to provide the correctpolarity to continue this direction of rotation. And similarly when thegun arrives in proximity to correspondence with the telescope the valve99 and switching device 104 would have become deenergized and thecontrol retransferred to the high speed fine control system.

It will be observed that in the modified systems of Figs. 2 and 3 themechanical switching apparatus 42-46 inclusive operated by the low speedreceiver 37 of Fig. 1 is eliminated and that this mechanical switchingapparatus is replaced in the modifications of Figs. 2 and 3 by electricvalve apparatus connected to produce the same controlling eiiect. Thiselimination of mechanical switching apparatus by the receiver of the lowspeed control system eliminates inaccuracies due to lost motion and wearof the mechanical switching arrangement.

Although in accordance with the provisions or the patent statutes thisinvention isdescribed as embodied in concrete form, it will beunderstood that the apparatus and connections shown and described aremerely illustrative and that the invention is not limited thereto sincealterations and modifications will readily suggest themselves to personsskilled in the art without departing from the true spirit of thisinvention and the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent or the UnitedStates, is:

- 1. In a follow-up system for driving an object into positionalagreement with a control device, an electric motor for driving thedriven object, means for eflecting fine control 0! said motor comprisingelectric valve apparatus provided with a control grid and meanscontrolled by positional disagreement of said control device and drivenobject for controlling said grid, and means for effecting coarse controlcomprising additional electric valve apparatus provided with a controlgrid, and means responsive to a predetermined amount of said positionaldisagreement for controlling said last mentioned grid and for renderingthe grid circuit of said fine control means inactive.

2. In a follow-up system for driving an object into positional agreementwith a control device, an electric motor for driving said driven object,means providing fine control of said motor comprising electric valveapparatus provided with a control grid, means biasing said grid to cutoil, means responsive to positional disagreement of said control deviceand driven object for supplying a voltage to overcome said bias andrender said valve apparatus active, and switching means actuated by saidvalveior controlling the supply of current to said motor, and meansproviding coarse control of said motor comprising additional electricvalve apparatus provided with a control grid, means biasing said lastmentioned grid to cut oil, means responsive to positional disagreementof said control device and driven object in excess of a predeterminedamount for overcoming said last mentioned bias and activating saidadditional valve apparatus, and switching means actuated by saidadditional valve apparatus for controlling the supply of current to saidmotor and rendering the grid circuit of said fine control inactive.

3. In a follow-up system for driving an object into positional agreementwith a control device, an electric motor for driving said driven object,fine control means for said motor comprising electric valve apparatusprovided with a control grid, means biasing said grid to cut oil, rotaryinduction apparatus operated at relatively high speed by said controldevice and driven object for supplying a voltage to said gridproportional to the positional disagreement of said objects forovercoming said bias and activating said valve apparatus, and switchingmeans actuated by said valve apparatus for controlling the flow ofcurrent to said motor, and coarse control means for said motorcomprising additional electric valve apparatus provided with a controlgrid, means biasing said last mentioned grid to cut oil, additionalrotary induction apparatusoperated at relatively low speed by saidcontrol device and driven object for supplying a voltage to overcomesaid last mentioned bias and activate said additional valve apparatuswhen the positional disagreement is in excess of a predetermined amountand switching means actuated by said additional valve apparatus forcontrolling the supply of current to said motor and rendering said firstmentioned switching means inactive.

MARTIN A. EDWARDS.

